Conventional concrete ranges from normal concrete for use in civil engineering and architectural constructions to high-fluidity concrete, high-strength concrete, mass concrete, underwater concrete, etc. depending upon the intended use, and is basically a material intended to be reinforced with reinforcing steel bars. Nowadays, however, there is a trend to employ so-called fiber reinforced concrete (FRC) in which short fibers are incorporated into the conventional concrete for the purpose of supplementing steel bar reinforcement, preventing corner defects of members, and preventing cracking due to drying shrinkage.
The aggregates blended in these concretes are composed of a fine aggregate and a coarse aggregate. In conventional concrete, a unit weight of aggregate contained in a unit volume of concrete is generally greater than a unit weight of powder (=unit weight of cement+unit weight of mineral admixture). For example, the ratio of the unit weight of aggregate to the unit weight of powder is given by 400 to 700% for the most commonly used type of concrete. It is about 250 to 300% even for powder-type high-fluidity concrete containing a large amount of powder.
Moreover, the largest particle diameter of coarse aggregate used in conventional concrete is limited most often to 20 mm or 25 mm in case of applying to general structures, and limited to 40 mm or 80 mm in case of applying to dams and the like. Thus, in conventional fiber reinforced concrete, the bonding mechanism between the fibers and concrete does not rely on mechanical bond through the aggregate mixed in concrete but relies on chemical adhesion and frictional force between cement hydrates (cement paste) and the fibers.
On the other hand, ultra-high-strength fiber reinforced concrete has been known which is obtained by mixing reinforcing fibers such as metallic fibers or organic fibers into a cementitious matrix that is obtained by mixing cement and pozzolanic reaction particles (pozzolanic material) into aggregate having a largest aggregate particle diameter of 1 to 2 mm (see Patent Documents 5 and 6, etc).
Ultra-high-strength fiber reinforced concrete as described above has such a characteristic that it can secure a certain level of tensile strength and toughness even after development of a crack, by combining fibers having high tensile strength with a cementitious matrix being dense and having ultra high strength. Specifically, this has been considered to be due to the exertion of a so-called bridging effect which allows the fibers to cover tensile force for the cementitious matrix when a crack is developed in the cementitious matrix as a result of tensile stress.
For this reason, unlike conventional reinforced concrete, ultra-high-strength fiber reinforced concrete as described above does not require reinforcement with reinforcing steel bars. Moreover, concrete structures built using ultra-high-strength fiber reinforced concrete as described above can achieve reduction in the thickness and the weight of its components.
Moreover, ultra-high-strength fiber reinforced concrete as described above can achieve significant improvement in durability because ultra-high-strength fiber reinforced concrete is often subjected to heat curing (steam curing), and denser hydrated cement particles are developed in a short time through a hydration process compared with normal moist curing. Further, after heat curing, ultra-high-strength fiber reinforced concrete has such characteristics that drying shrinkage becomes almost zero, and a creep coefficient is significantly decreased.
The cementitious compositions disclosed in Patent Document 1 and Patent Document 2 have almost the same mix proportion except the type of cement. These Patent Documents are different in that the type of cement in the cementitious composition is ordinary Portland cement, high-early-strength Portland cement, or moderate-heat Portland cement in Patent Document 1, while it is low-heat Portland cement in Patent Document 2. The cementitious compositions of Patent Documents 1 and 2 are characterized in that the effects of improvement in fluidity, shortening of setting time, improvement in mixing properties, etc. are obtained by blending limestone powder having a specific grading distribution. According to Patent Documents 1 and 2, the improvement in fluidity or mixing properties cannot be achieved only by the adjustment of the fineness (Blaine specific surface area) of limestone powder, but it is indispensable that limestone powder should have a specific grading distribution.
However, the cementitious compositions disclosed in Patent Documents 1 and 2 are cementitious compositions for forming conventional concrete materials, and are not targeted for an ultra-high-strength cementitious matrix of ultra-high-strength fiber reinforced concrete which does not contain coarse aggregate as mentioned above. For this reason, although these cementitious compositions contain Portland cement, silica fume, and limestone powder, the effects of the limestone powder under conditions where a mineral admixture such as a pozzolanic material is blended are neither described nor suggested.
Both Patent Document 3 and Patent Document 4 are the documents on ultra-high-strength fiber reinforced concrete. Fibers contained in the latent hydraulicity composition of Patent Document 3 are organic fibers and carbon fibers, while fibers in Patent Document 4 are metallic fibers. Thus, these Patent Documents are different in terms of fibers, but are common in the cementitious matrix. The cementitious matrices disclosed in these documents are each composed of cement, fine particles, and two kinds of inorganic particles, in which a specific surface area and mix proportions by weight are specified to each of the materials.
Moreover, Patent Documents 3 and 4 each disclose a latent hydraulicity composition developed for the purpose of improving fluidity and segregation resistance, and improving mechanical properties such as compressive strength after curing. With respect to these disclosures, the literatures do not show the improvement in performance based on chemical reaction of materials blended, but describe the performance improvement in fluidity and segregation resistance by paying attention to the grading distribution or Blaine specific surface area of materials. Further, improvement in compressive strength after curing is also described by paying attention to the fact that the constituent materials are mixed by densest packing. These Patent Documents neither describe nor suggest the effects of the cementitious matrix obtained by mixing cement, silica fume, at least one pozzolanic material, and limestone powder on early strength, low shrinkage, low heat of hydration, high fluidity, high tensile strength, high toughness, and etc.
Moreover, Patent Documents 5 to 8 each disclose a composition of a cementitious matrix composed of cement and particles which undergo pozzolanic reaction. Further, metallic fibers, organic fibers, composite fibers obtained by combining organic fibers with metallic fibers, or the like are contained as fibers for reinforcing these cementitious matrices. The ultra-high-strength fiber reinforced concretes disclosed in these documents are characterized in that they allow improvement in fluidity, improvement in segregation resistance, improvement in durability by densifying a cementitious matrix, and improvement in mechanical characteristics after curing. Mixing of pozzolanic reaction particles leads to a state where pozzolanic reaction caused by the presence of cement can be expected, which allows improvement in mechanical properties after curing to be achieved. Moreover, a pozzolanic material also achieves the grading adjustment function of the components of a cementitious matrix composition. Accordingly, improvement in fluidity, improvement in segregation resistance, and densification of a cementitious matrix can be achieved. However, these Patent Documents neither describe nor suggest the effects generated by mixing limestone powder in addition to pozzolanic reaction particles and the achievement of high tensile strength and high toughness by improving the bonding performance between reinforcing fibers and a cementitious matrix.